Phenylketonuria (PKU), due to phenylalanine hydroxylase (PAH) deficiency, is treated by dietary restriction of phenylalanine (Phe). Treatment of PKU with Phe-restricted diet has been difficult and often not possible to achieve the desired level of blood Phe for many patients. Reports indicate poor school performance, loss of IQ, and white matter deterioration in the brain when diet is relaxed. Furthermore, there is no agreed upon criteria for follow up and what blood Phe levels need to be pursued in older children and adults. Another complicating factor is pregnancy with PKU, which is at risk for offspring with neurological deficits and congenital heart defects. Recently, some patients with PAH deficiency have been found to respond to high doses of tetrahydrobiopterin (BH4). Tetrahydrobiopterin is a cofactor required for PAH activity. The reports of PAH deficiency responding to BH4 suggest Km mutations of PAH while the metabolism of BH4 is normal. Analysis of PAH in these patients showed various mutations, suggesting that there may be a wide range of PAH mutations that should respond to BH4. This pilot study is intended to investigate, over a period of two years, patients with PKU and their response to BH4 loading. The metabolism of BH4 will be studied, mutations of PAH will be determined, and the response to BH4 will be studied by monitoring blood Phe levels, in situ directed mutagenesis, enzyme expression, and enzyme kinetics in the presence of varying levels of BH4. Earlier studies of in vitro expression of PKU mutations were not aimed to identify mutations that respond to BH4, so such data are not available on approximately 400 mutations of PAH. Success of these studies will lead to new research on the treatment for many patients with PKU, and for a systematic new study of the mutations of PAH in order to identify which mutations respond to BH4. Treatment with BH4 should lead to a better outcome for PKU patients and for maternal PKU.